The widespread presence of same-sex sexual behaviour (SSB) has long been thought to pose an evolutionary conundrum, as participants in SSB suffer the cost of failing to reproduce after expending the time and energy to find a mate. The potential for SSB to occur as part of an optimal strategy has received less attention, although indiscriminate sexual behaviour may be the ancestral mode of sexual reproduction. Here, we build a simple model of sexual reproduction and create a theoretical framework for the evolution of indiscriminate sexual behaviour. We provide strong support for the hypothesis that SSB can be maintained by selection for indiscriminate sexual behaviour, by showing that indiscriminate mating is the optimal strategy under a wide range of conditions. Further, our model suggests that the conditions that most strongly favour indiscriminate mating were probably present at the origin of sexual behaviour. These findings have implications not only for the evolutionary origins of SSB, but also for the evolution of discriminate sexual behaviour across the animal kingdom.

长期以来，同性性行为 (SSB) 的广泛存在一直被认为是一个进化难题，因为 SSB 的参与者在花费时间和精力寻找配偶后，要承受无法繁殖的代价。SSB 作为最佳策略的一部分发生的可能性较少受到关注，尽管不分青红皂白的性行为可能是有性生殖的祖先模式。在这里，我们建立了一个简单的有性生殖模型，并为不分青红皂白的性行为的演变创建了一个理论框架。我们通过证明不加区别的交配是在各种条件下的最佳策略，为通过选择不加区别的性行为来维持 SSB 的假设提供了强有力的支持。更远，我们的模型表明，最有利于不分青红皂白交配的条件可能存在于性行为的起源处。这些发现不仅对 SSB 的进化起源有影响，而且对整个动物王国歧视性行为的进化也有影响。